today i managed to play with l3 flavors.
i wrote a crude patch to implement midonet flavor. 
a good news is it's somehow working.
a bad news is it has a lot of issues, as you can see in TODO comments
in the patch.
given these issues, now i tend to think it's cleaner to introduce
ml2-like precommit/postcommit driver api (or its equivalent via
callbacks) rather than using these existing notifications.
how do you think?
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